These leaks can reduce the heat transferred to the steam and increase pumping requirements. However, such leaks are often easily repaired, saving 2% to 5% of the energy formerly used by the boiler . This measure differs from flue gas monitoring in that it consists of a periodic repair based on visual inspection. The savings from this measure and from flue gas monitoring are not cumulative, as they both address the same losses. Reduction of excess air. When too much excess air is used to burn fuel, energy is wasted because excessive heat is transferred to the air rather than to the steam. Air slightly in excess of the ideal stochiometric fuel/-to-air ratio is required for safety and to reduce emissions of nitrogen oxides , but approximately 15% excess air is generally adequate . Most industrial boilers already operate at 15% excess air or lower, and thus this measure may not be widely applicable . However, if a boiler is using too much excess air, numerous industrial case studies indicate that the payback period for this measure is less than 1 year . For example, at a U.S. DOE sponsored energy audit of a Land O’Lakes dairy facility in Tulare, California, it was estimated that by reducing excess oxygen from 4.5% to 3.0%, the facility would reduce its natural gas costs by $113,000 per year while still meeting stringent NOX emissions limits . As a rule of thumb, the Canadian Industry Program for Energy Conservation estimates that for every 1% reduction in flue gas oxygen, boiler efficiency is increased by 2.5% .
Properly sized boiler systems. Designing the boiler system to operate at the proper steam pressure can save energy by reducing stack temperature, round plant pot reducing piping radiation losses, and reducing leaks in steam traps. This measure is particularly important in fruit and vegetable processing facilities, where due to the seasonality of production, large boilers can often be run at low capacity during the off season, which can result in significant energy losses. In a study done in Canada on 30 boiler plants, savings from this measure ranged from 3% to 8% of total boiler fuel consumption . Savings were greatest when steam pressures were reduced below 70 pounds per square inch . One industrial case study has shown that correct boiler sizing led to savings of $150,000 at a payback period of only 2.4 months . However, costs and savings will depend heavily on the current boiler system utilization at individual plants. Improved boiler insulation. It is possible to use new materials, such as ceramic fibers, that both insulate better and have a lower heat capacity . Savings of 6% to 26% can be achieved if improved insulation is combined with improved heater circuit controls. Due to the lower heat capacity of new materials, the output temperature of boilers can be more vulnerable to temperature fluctuations in the heating elements . Improved boiler process control is therefore often required in tandem with new insulation to maintain the desired output temperature range. At a U.S. DOE sponsored assessment of a Land O’Lakes dairy facility in Tulare, California, it was found that by improving insulation on the facility’s steam header, boiler economizer, and process hot water tank, the company could save nearly $35,000 per year in reduced boiler fuel costs . Boiler maintenance.
A simple maintenance program to ensure that all components of a boiler are operating at peak performance can result in substantial savings. In the absence of a good maintenance system, burners and condensate return systems can wear or get out of adjustment. These factors can end up costing a steam system up to 30% of initial efficiency over two to three years . On average, the energy savings associated with improved boiler maintenance are estimated at 10%. Improved maintenance may also reduce the emission of criteria air pollutants. Fouling on the fire side of boiler tubes or scaling on the water side of boilers should also be controlled. Fouling and scaling are more of a problem with coal-fed boilers than natural gas or oil-fed boilers . Tests reported by CIPEC show that a fire side soot layer of 0.03 inches reduces heat transfer by 9.5%, while a 0.18 inch soot layer reduces heat transfer by 69% . For water side scaling, 0.04 inches of buildup can increase fuel consumption by 2% .Flue gas heat recovery. Heat recovery from flue gas is often the best opportunity for heat recovery in steam systems . Heat from flue gas can be used to preheat boiler feed water in an economizer. While this measure is fairly common in large boilers, there is often still room for more heat recovery. The limiting factor for flue gas heat recovery is that one must ensure that the economizer wall temperature does not drop below the dew point of acids contained in the flue gas . Traditionally, this has been done by keeping the flue gases exiting the economizer at a temperature significantly above the acid dew point. In fact, the economizer wall temperature is much more dependent on feed water temperature than on flue gas temperature because of the high heat transfer coefficient of water. As a result, it makes more sense to preheat feed water to close to the acid dew point before it enters the economizer.
This approach allows the economizer to be designed so that exiting flue gas is just above the acid dew point. Typically, one percent of fuel use is saved for every 45°F reduction in exhaust gas temperature . At the Odwalla Juice Company’s facility in Dinuva, California, the installation of an economizer was expected to save over $21,000 per year in energy costs and over 4,000 MBtu of boiler fuel per year . Odwalla’s expected payback period for the economizer was just 10 months. McCain Foods, a major producer of frozen French fried potatoes, installed an economizer at its Scarborough, England, facility as part of a plant-level heat recovery project in 1995. The new economizer saved the facility 67 therms of natural gas per hour, leading to energy savings of £67,000 per year with a simple payback period of 2.5 years . Similar results were expected at Schneider Foods, a packaged and frozen meats company in Ontario, Canada. In 2005, the company installed a dual-stage economizer, which heats both boiler feed water and boiler makeup water with heat recovered from flue gas, leading to savings of about $225,000 per year and a payback period of less than two years . Condensate return. Reusing hot condensate in boilers saves energy, reduces the need for treated boiler feed water, and reclaims water at up to 100°C of sensible heat. Typically, fresh feed water must be treated to remove solids that might accumulate in the boiler; however, returning condensate to a boiler can substantially reduce the amount of purchased chemical required to accomplish this treatment. The fact that this measure can save substantial energy costs and purchased chemicals costs often makes building a return piping system attractive. A 2005 study of seven different fresh fruit and vegetable processing plants in California estimated a payback period for this measure ranging from approximately two to three years . Blow down steam recovery. When water is blown from a high-pressure boiler tank, the pressure reduction often produces substantial amounts of steam. This steam is typically low grade, but can be used for space heating and feed water preheating. The recovery of blow down steam can save around 1% of boiler fuel use in small boilers . In addition to energy savings, blow down steam recovery may reduce the potential for corrosion damage in steam system piping.Green Giant of Canada, a manufacturer of frozen and canned vegetables, installed a shell and tube heat exchanger to recover heat from boiler blow down. This measure led to annual energy savings of roughly $1,500 with a payback of approximately 2 years . Boiler replacement. Substantial efficiency gains can often be realized by replacing old boilers with new, round garden pot higher efficiency models. In particular, the replacement of inefficient coal fired boilers with natural gas-fired boilers is a sound strategy for reducing boiler fuel costs while also reducing emissions of air pollutants. Valley Fig, a manufacturer of fig pastes and concentrates in Fresno, California, replaced their old and inefficient 300 boiler horsepower fire tube boiler in 2004 in order to meet stringent NOX emissions limits. The 300 bhp boiler was replaced with two smaller and more efficient 100 bhp boilers, which not only allowed them to meet the facility’s steam demands while lowering NOX emissions, but also reduced their natural gas costs by 8% to 10% . Additionally, Valley Fig received a $16,000 rebate check from Pacific Gas & Electric for improved fuel efficiency. Direct contact water heating. In direct contact water heaters, water is sprayed downward through a vertical chamber that serves as a flue for combustion gases.
Because the hot combustion gases heat the water directly, this water heating system is more efficient than traditional boilers. Hot water is collected in a storage tank while the combustion gases exit the system at near-ambient temperatures. Since water does not contact the burner flames, complete combustion occurs before the gases heat the water. Thus, water quality is maintained to a level that is appropriate for food processing operations . Additionally, direct-contact water heaters can operate at atmospheric pressure, which avoids the safety hazards and insurance premiums that can come with pressurized boiler operation. One commercially-available direct-contact water heater by Kemco Systems, Inc., offers water heating efficiencies of up to 99.7%, which is a significant improvement compared to the 60% to 75% efficiencies achievable with traditional water heating technologies . Approximately 3,000 Kemco direct-contact water heaters are said to be in operation worldwide, with average payback periods ranging from one to two years. Another commercially-available direct-contact water heating system by QuickWater was installed at Golden Temple, a natural foods manufacturing company based in Oregon, in 2003. Golden Temple’s annual energy savings for water heating were estimated at 22%, with annual energy cost savings totaling around $2,300 . Additionally, the direct-contact water heater was said to offer a smaller footprint than traditional systems as well as a longer life .Steam and hot water distribution systems are often quite extensive and can be major contributors to energy losses within a fruit and vegetable processing plant. Energy efficiency improvements to steam distribution systems are primarily focused on reducing heat losses throughout the system and recovering useful heat from the system wherever feasible. The following measures are some of the most significant opportunities for saving energy in industrial steam distribution systems. Improved distribution system insulation. Using more insulating material or using the best insulation material for the application can save energy in steam systems. Crucial factors in choosing insulating material include low thermal conductivity, dimensional stability under temperature change, resistance to water absorption, and resistance to combustion. Other characteristics of insulating material may also be important depending on the application, such as tolerance of large temperature variations, tolerance of system vibrations, and adequate compressive strength where the insulation is load bearing . Industrial case studies indicate that the payback period for improved insulation is typically about one year . The S. Martinelli Company, an apple juice manufacturer in Watsonville, California, found that insulating steam distribution lines not only led to energy savings, but also reduced the amount of heat inadvertently released to interior spaces . Insulation maintenance. It is often found that after heat distribution systems have undergone some form of repair, the insulation is not replaced. In addition, some types of insulation can become brittle or rot over time. As a result, a regular inspection and maintenance system for insulation can also save energy . Steam trap improvement. Using modern thermostatic element steam traps can reduce energy use while also improving reliability. The main efficiency advantages offered by these traps are that they open when the temperature is very close to that of saturated steam , purge non-condensable gases after each opening, and are open on startup to allow a fast steam system warm-up. These traps also have the advantage of being highly reliable and useable for a wide variety of steam pressures . Steam trap maintenance. A simple program of checking steam traps to ensure that they are operating properly can save significant amounts of energy for very little money.